Reproduction steps:
1. Open the attached project ("case_1239413-SphereCast.zip")
2. Open the repro scene ("SphereCast")
3. Enter Play Mode
4. Inspect Console and rays inside the scene

Expected result: No logs are shown (OverlapCapsule and SphereCast finds same info)
Actual result: SphereCast doesn't detect mesh collisions even though OverlapCapsule over the same region founds them

Reproducible with: 2018.4.24f1, 2019.4.2f1, 2020.1.0b14, 2020.2.0a15

Notes:
1. Almost any change in parameters one way or the other will trigger the expected behaviour.
2. Similarly, increasing or decreasing the spherecast radius by a tiny amount will both produce hits, showing the issue is related to the angle/location of where the first hit would have been.
3. Setting the custom mesh to Convex mode produces a hit, so this issue might only be present in concave mode.